In a two way radio communication system there are often a number of communication resources, for example frequencies, time periods, coding schemes, that are available for allocating to communication system user's to enable the users to set up a communication link. In some communication systems, the radio users transmit and receive on the same radio frequency. In such a system, assuming no geographical reuse of the frequency, only one call can be established between at least two radio units at any one time. It is possible, and often implemented in multi-site systems, that the same communication resources are re-allocated on a geographical basis to some users as long as they do not interfere with existing users using that particular resource, for example where any potential interfering signal level from new users would be too low to be detected by the existing users.
In two-way radio communications systems, communications are typically performed via individual calls between two users or via group calls where users communicate to a number of other users in a particular communication group. A problem exists with the reuse strategy in the individual call scenario, as highlighted in FIG. 1. Portable radio unit 10 has entered into communication on a frequency resource with portable radio unit 12 and creating a coverage range for its transmissions of geographic area 14. Portable radio unit 16 is within that geographic area 14 and in close proximity to portable radio unit 10. Therefore, portable radio unit 16 receives a strong interfering signal from portable radio unit 10 on the frequency resource, whenever portable radio unit 10 transmits, such that it can not receive a communication from portable radio unit 18 on that resource. Portable radio unit 18 has no knowledge of the problems that exist for portable radio unit 16 on that frequency resource and may select that frequency resource as the chosen frequency for communicating with portable radio unit 16. As such a communication will not be completed.
The decision process for choosing a communication resource, in either a group call or individual call scenario, typically involves monitoring the resources available for communications, such as frequencies, and the calling radio selecting and establishing a call on a free resource, as seen by the calling radio. This situation does not arise in trunked communication systems where resources are allocated by a system controller, according to the current needs of users on the system, and the details of the chosen resource transmitted to the respective users via say, a signalling control channel or signalling time-slot.
A further difficulty arises when a radio unit wishes to communicate to a group of users, where the chosen communication resource may not be ideal or even available to all users. Each group may include a number of radio units using a common identification signal, with each radio unit monitoring the resources, waiting for their individual identification signal or group ID to be transmitted, in order to then participate in a subsequent call. Transmission of the common identification signal may occur via a polling technique, where all of the users in the desired group are polled as to the resources available at their particular location. In the digital short range radio (DSRR) standard the calling radio selects a frequency resource available at its location and transmits the details of this channel to the users the calling radio wishes to communicate with.
In a time division multiple access (TDMA) communication system time and frequency resources are allocated to users in trunked and/or non-trunked communications, for example where each frequency includes four time slots for use by four separate users. In non-trunked operation, i.e. direct mode operation (DMO) in the European TETRA standard, the radio units may communicate on a particular time slot dedicated for this mode of operation.
There are a number of disadvantages of the above described methods for the allocation of communication resources, primarily including the fact that the optimum resource for both the calling and called radio(s) is not necessarily selected.
This invention seeks to provide a two way communication system, a method of operating the two way communication system and a radio unit for use in the two-way communication system which mitigates at least some of the above mentioned disadvantages.